plasmid
/ PLAZ-mid /
A bacterium keeps its essential instructions on its main chromosome, much like a phone keeps its operating system in built-in storage. But it can also carry small, optional 'apps' of extra DNA that aren't part of the main chromosome and can be picked up, dropped, or shared. A plasmid is one of those optional add-ons: a small, usually circular loop of DNA that lives separately from the main genome, mostly in bacteria.
Physically, a plasmid is a tiny ring of double-stranded DNA, carrying just a handful of genes, that copies itself independently of the host's main chromosome. The genes it carries are often not strictly necessary for survival but give the bacterium a useful extra trick under certain conditions — most famously, resistance to an antibiotic, or the ability to digest an unusual food source. Crucially, plasmids can be passed not only to a cell's offspring but sideways, directly between living bacteria, a process called conjugation.
Plasmids matter enormously for two reasons. In nature, they are a major highway for spreading antibiotic resistance: one resistant bacterium can hand a resistance plasmid to unrelated bacteria, even of different species. In the lab, scientists exploit plasmids as the workhorse tool of genetic engineering — they insert a gene of interest into a plasmid, slip it into bacteria, and let the bacteria mass-produce the gene's product, which is how human insulin and many other medicines are made.
Today's insulin is made by inserting the human insulin gene into a plasmid, putting that plasmid into bacteria or yeast, and letting the microbes churn out the protein in giant fermentation tanks.
A bacterial DNA ring repurposed to manufacture medicine.
A plasmid is not the bacterium's main chromosome and is usually non-essential — the cell can survive without it, which is exactly why plasmids can be gained, lost, and swapped so freely.